1. Field of the Invention
The present invention relates to a thermal insulation engine and particularly, a thermally insulated engine structure having a combustion chamber section made of a ceramic or equivalent composite material which has a higher resistance to heat and thereby increasing the thermal efficiency during operation at a high temperature without the need for an extra cooling service.
2. Description of the Prior Art
A variety of thermal insulation engines have been developed having cylinders and pistons in the combustion chamber section made of highly heat-resistant ceramic or other composite materials rather than conventional metals to provide a higher thermal insulation structure. The thermal efficiency during operation at high combustion temperatures will be increased without the use of any cooling system.
In general, the combustion chamber of such a thermal insulation engine has a ceramic inner wall covered at an outer side with a low thermal conductive material, thus comprising a composite construction. Accordingly, the thermal insulation engine can offer a higher thermal insulation effect with its structure. This type of thermal insulation engine includes a zirconia coated combustion chamber which is best known.
The engine of the foregoing type is however a heat regist type combustion engine. However the quantity of heat removed from the combustion gas to the combustion chamber is large. Therefore the escape of heat can hardly be prevented.
As the combustion chamber section of the engine is made of a highly heat resistant material, such as silicon nitride with its protective cladding of a low thermal conductive material, the radiation of heat is substantially reduced.
It is known that the transfer of heat mass Q across the wall of a combustion chamber is calculated from: EQU Q=As-Ki(Tg-Ta) (1)
where As is the surface area of the combustion chamber, Ki (i=1 or 2) is the thermal transmittance, Tg is the temperature of gas, and Ta is the temperature of air or water.
If K1 is a thermal transmittance of the combustion chamber, and the combustion chamber consists of a composite material made of a stainless steel coated with a zirconia ceramicmaterial, the equation for Ki is expressed as: EQU K1=1/(1/ag+d1/kpz+d2/kst+1/ac) (2)
It is now assumed that ag is a coefficient of heat transfer determined by the state of the gas in the cylinder and commonly, 250 kcal/(m .degree.C. h), ac is a coefficient of heat transfer from cooling water to cylinder body and commonly, 5000 kcal/(m .degree.C. h), kpz is a thermal conductivity of zirconia as 5 kcal/(m .degree.C. h), and kst is a thermal conductivity of stainless steel as 40 kcal/(m .degree.C. h). When those values are substituted for the terms in the equation (2), K1 is as high as approximately 200 kcal/(m .degree.C. h).
If the construction is shifted to a double insulation structure which comprises a by heat-proof combustion chamber surrounded by air gaps, its thermal transmittance K2 is expressed as: EQU K2=1/(1/ag+d1/kpz+d2/ksn+d3/kst+1/ac) (3)
As indicated above, ag is a coefficient of heat transfer determined by the state of the gas in the cylinder and commonly, 250 kcal/(m .degree.C. h), ac is a coefficient of heat transfer from cooling water to cylinder body and commonly, 5000 kcal/(m .degree.C. h), kpz is a thermal conductivity of zirconia as 5 kcal/(m .degree.C. h), and kst is a thermal conductivity of stainless steel as 40 kcal/(m .degree.C. h). Therefore, K2 is approximately 88 kcak/(m .degree.C. h) as calculated from the equation (3).
As apparent, the thermal transmittance of the conventional thermal insulation engine equipped with the zirconia coated combustion chamber remains high and will hardly increase the thermal efficiency of the engine.